Impairments of selective and divided attention are two of the most reliable cognitive deficits associated with aging. Information concerning the neurobiological mechanisms and neural circuits involved in attention has started to accumulate in the past decade. However, relatively little is known about how these mechanisms and circuits change with age. The proposed studies examine whether aged rats have impaired selective and divided attention, as demonstrated in aged humans. Furthermore, the age-related changes in the basal forebrain pathway to the frontal cortex are examined. These brain regions are known to be important for selective and divided attention in young adults. Finally, the relationship between attentional impairments and anatomical changes are explored by correlational analyses. The hypothesis is that selective and divided attention are impaired in aged rats due to the degeneration of basal forebrain neurons that project to the frontal cortex. Aged and young adult rats are tested in 3 versions of a two-choice reaction time task to assess selective and divided attention. The conditions examined are single task which measures performance on a single 2-choice discrimination, selected task which measures performance on a single 2-choice discrimination with distractors, and dual task which measures performance on two simultaneous 2-choice discriminations. Performance differences between selected and single task conditions provide a measure of selective attention ability. Performance differences between dual and single task conditions provide a measure of divide attention ability. After behavioral testing, brains are processed for anatomical analyses. Basal forebrain neurons that project to the frontal cortex are labeled with retrograde tracers. These neurons are subdivided into cholinergic and calbindin-containing neurons because these neurons represent populations that may degenerate during aging. Quantitative analyses of these neuronal populations are made using stereology. Age-related changes in neuronal populations are correlated with attentional impairments. These studies are important because they start to identify the neural circuits that may be responsible for the age-related impairments of attention and provide information that is essential for developing treatments aimed at preventing the cognitive decline associated with aging.